Many different means of security are available to prevent duplication of printed commercial documents such as special papers (water marked paper) and special inks (fluorescent inks and other optically variable inks) which form latent images or images that change color.
The use of latent images as a security measure is well known. To be useful as a security measure, latent images must be well camouflaged but readily and easily viewable to the user, preferably by a simple procedure. An example of such a latent image is described in U.S. Pat. No. 5,468,581, which is formed when printing documents using an intaglio process. The latent image is overprinted on the visible image such that the latent image is visible when the document is tilted and viewed at an angle. The latent image is caused by the variation of the slight shadow from the raised ink pattern formed by the intaglio process or other printing method which produces raised ink patterns.
Optically variable inks have been used to provide latent images and images which change color when exposed to a light source other than ambient light. These optically variable inks allow for non-destructive testing of the security feature allowing the printing of such inks to be monitored. Such optically variable inks typically contain a fluorescent compound which responds to infrared or ultraviolet light. An example of an aqueous printing ink for jet printing which fluoresces under ultraviolet radiation is described in U.S. Pat. No. 4,153,593. The dyes described in this reference are water soluble and include fluorescein, eosine dyes and Rhodamine dyes. Representative disclosures of other inks include U.S. Pat. No. 4,328,332, issued to Hayes et al. on May 4, 1982, and U.S. Pat. No. 4,150,997, issued to Hayes on Apr. 24, 1979. While the use of fluorescent inks and dyes has been effective, with the advent of todays personal computers and color copiers, security measures relying on these optically variable inks have been overcome, particularly where records are only casually inspected, such as sales receipts and transaction records. Therefore, it is desirable to provide new means for forming latent images on printed commercial documents.
Krutak et al. describe the use of near infrared fluorescent (NIRF) compounds used in polyester-based and polyester-amide based coatings and ink compositions which are used for marking articles for identification/authentication purposes, in U.S. Pat. No. 5,292,855, issued Mar. 8, 1994, U.S. Pat. No. 5,423,432, issued Jun. 13, 1995, and U.S. Pat. No. 5,336,714, issued Aug. 9, 1994. Krutak et al. also disclose tagging thermoplastic containers and materials with near infrared flourescent compounds in U.S. Pat. No. 5,461,136, issued Oct. 24, 1995, U.S. Pat. No. 5,397,819, issued Mar. 14, 1995, and U.S. Pat. No. 5,703,229, issued Dec. 30, 1997. The use of near infrared flourescent compounds as a security ink in thermal transfer printing has also been disclosed in International application WO 97/32733, published Sep. 12, 1997, wherein an image is formed by thermally transferring ink from a ribbon to paper.
Escano et al., U.S. Pat. Nos. 5,614,008 and 5,665,151, also disclose inks containing NIRF compounds.
Kaule et al., U.S. Pat. Nos. 4,452,843 and 4,598,205, disclose rare earth metal luminophores which absorb in the visible region and optionally the near-infrared region and can be excited in substantial portions of the visible or near IR-region.
Yoshinaga et al., U.S. Pat. No. 5,503,904, disclose recorded media with an invisible identification mark composed regions of high reflectance and low reflectance in the same near infrared region. Near infrared coloring materials are said to comprise xanthene, oxazine, thiazine, polymethine and stryl compounds.
Direct thermal paper is a thermosensitive recording material on which print or a design is obtained by the application of heat energy, without an ink ribbon. Thermal paper comprises a base sheet and a coating, and like other coated papers, the coating is applied to give new properties to the base sheet. However, a major distinction in thermal paper from other coated papers is that special color forming chemicals and additives are present in the coatings such that when heat is applied by a thermal head, the color forming chemicals react to develop the desired print or image.
The most common type of thermal coating is the dye-developing type system. The three main color producing components in a dye developing-type thermal paper are colorless dye (color former), a bisphenol or an acidic material (color developer) and sensitizer. These solid materials are reduced to very small particles by grinding and are incorporated into a coating formulation along with any optional additives such as pigments, binders and lubricants. This coating formulation is then applied to the surface of paper or other support system using various types of coating application systems and dried. Images are formed on the coated surfaces by the application of heat to melt and interact the three color producing materials.
Where security features from optically variable inks are desired for thermal paper, the optically variable inks must not pre-react the reactive components within the thermosensitive coating of the thermal paper to detract from the thermal papers printing performance. Certain chemicals can adversely affect and degrade the performance of the thermosensitive coating and should be avoided such as some organic solvents (ketones), plasticizers (polyethylene glycol type), amines (ammonia) and certain oils (soy oil). The coating solutions for NIRF compounds typically contain amines and other compounds which the thermosensitive coatings react with. It is desirable to provide a thermal paper with NIRF incorporated therein which does not result in pre-reacted components in the thermosensitive coating.
To protect thermal paper from environmental conditions, and premature coloration from handling, a number of developments have been made. One is to produce a barrier or protection layer on top of the thermal coating (see U.S. Pat. Nos. 4,370,370; 4,388,362; 4,424,245; 4,444,819; 4,507,669; and 4,551,738). Another approach is to encapsulate the reactive components in microcapsules which rupture or are permeable when exposed to heat. See U.S. Pat. Nos. 4,682,194; 4,722,921; 4,742,043; 4,783,493; and 4,942,150. These protection measures are not reliable in preventing premature coloration of the thermosensitive layer when exposed to a NIRF coating solution.